Y-Axis positions of barplot and base plot do not match

I was trying to plot a climate diagram and ran into the following problem:
After using barplot(...) for precipitation I superimposed another plot for the temperature. It is necessary for climate diagrams that the two y-axes (mm, °C) align at zero and that the precipitation/temperature ratio is 2:1 (e.g. 20mm precipitation corresponds to 10°C).

The problem: barplot(...) draws the axis to the plot's box while plot(...) leaves some space between the box and the axis margins.

Here is a simplified example. From the grid lines you see that the 0-values do not align: